Recently, strong demands have arisen for a higher density and higher integration degree of semiconductor integrated circuits. In addition, to improve the productivity of semiconductor elements, the exposure time is required to be shorter. In lithography, for processing a circuit pattern, an exposure apparatus using, as a light source, far UV light or an excimer laser beam capable of obtaining high illuminance at a short wavelength is generally used for micropatterning and a shorter exposure time.
This technique readily activates a gas around the optical elements of an illumination optical system or projection optical system as it is irradiated with light. For this reason, there is a high probability of the surfaces of the optical elements becoming contaminated. A method of preventing contamination has been proposed in which the optical elements of an illumination optical system or projection optical system are accommodated in a closed vessel or the like, and the vessel is filled or replaced with clean dry air or an inert gas, thereby preventing contamination of the internal optical elements.
For an exposure apparatus for projecting and exposing a finer circuit pattern using an ArF excimer laser beam (wavelength: 193 nm), a method has been proposed in which glass materials of at least two types, such as molten silica and fluorite, are used to form the refraction optical elements of a projection optical system, the space in the projection optical system is divided into at least two spaces, and at least one of these spaces is filled with a gas having a refractive index different from that of a gas in the other space, thereby improving the characteristics (optical performance) of the projecting lens.
For a projection optical system, the gas in the optical system must be purged for several purposes as described above. The present invention is applied to improve the optical performance, especially in an apparatus for projecting and exposing a micropattern, by dividing the internal space of a projection optical system into two or more spaces and filling the respective spaces with gases having different refractive indices.
In this case, the boundary portion between the spaces is formed from a refraction optical element (lens) and a lens holding member. To specially shield the boundary portion, an O-ring or adhesive (or filler) is used.
When an O-ring made of rubber (e.g., fluororubber) is used, sufficient sealing properties are obtained in general. However, in the arrangement shown in FIG. 1, when a sufficient crush amount is given to the O-ring, excessive deformation or internal stress is produced in the lens through the lens holding member, resulting in degradation in optical performance of the projecting lens.
If an adhesive (or filler) 12 is used, as shown in FIG. 7, a degassed component from the adhesive or filler 12, or a chemical reaction component between the degassed component and exposure light or heat sticks to the lens, resulting in degradation in optical performance of the projecting lens.
If a simple fitting or an O-ring with poor sealing properties is used, the gases in the respective spaces mix to change the refractive index of each gas. For this reason, predetermined optical performance may not be obtained.
Hence, a purge method and structure capable of sufficiently satisfying the optical performance of the projecting lens using a lens barrel structure capable of preventing any change and stress generation in a lens at the boundary portion between purge spaces must be established.